Synthesis of magnetic Fe3O4/graphene aerogel for the removal of 2,4-dichlorophenoxyacetic acid herbicide from water

Graphene-based aerogels are among the lightest materials in the world and have been extensively studied for environmental remediation. In this work, an Fe3O4/graphene aerogel material was synthesized using the co-precipitation method. The prepared material was characterized using X-ray diffraction (XRD), scanning electron microscopy/X-ray energy dispersive spectroscopy (FESEM/EDX), infrared spectroscopy (FT-IR), and vibration sample magnetization (VSM). The results showed that the Fe3O4 nanoparticles with a particle size of less than 100 nm were well-distributed on the surface of the graphene aerogel. The prepared Fe3O4/graphene aerogel showed effective removal of 2,4-D herbicide from the aqueous solution with a maximal adsorption capacity of approximately 42.918 mg g−1. The adsorption isotherms and kinetics were investigated to study the adsorption behaviour of the resultant material. The saturation magnetism value of the aerogel was determined to be about 20.66 emu g−1, indicating that the adsorbent could be easily collected from the solution using an external magnet. These results implied that the prepared Fe3O4/graphene aerogel could be a promising adsorbent for the removal of 2,4-D herbicide from water.


Introduction
There is no naturally occurring 2,4-dichlorophenoxyacetic acid (2,4-D) in the environment.2][3] Its purpose is to effectively eliminate weeds on land and in aquatic environments. 4,5The substance is a colorless to light brown solid with no smell, and it has a higher density than water, causing it to sink.2,4-D is a chlorophenoxyacetic acid with chlorine substituting the ring hydrogens at positions 2 and 4. It functions as a synthetic auxin, a substance that mimics the effects of a plant hormone, as well as a defoliant, an agrochemical (a chemical used in agriculture), an EC inhibitor (explicitly inhibiting the enzyme shikimate dehydrogenase), an environmental pollutant, and a phenoxy herbicide (a type of herbicide that contains the phenoxy group). 4,6-92,4-D is a widely used herbicide that acts throughout the entire plant system and is effectively manages broadleaf weeds. 8,10It is the most extensively employed herbicide globally and the third most frequently utilized in North America. 11,12Oen used as an addition to plant cell culture media like MS medium and in laboratory settings for plant research, 2,4-D is a notable articial auxin. 13,146][17] 2,4-D was a constituent of Agent Orange, the pesticide that was extensively utilized during the Vietnam War.While 2,4-D accounted for 50% of the composition of Agent Orange, the health consequences of Agent Orange are mainly attributed to the presence of dioxin pollutants produced during its manufacturing process rather than 2,4-D itself.Methods have been studied to remove 2,4-dichlorophenoxyacetic acid, including degradation, 18,19 adsorption, [20][21][22][23][24] ltration, 25 electrochemically assisted adsorption, 26 biodegradation, [27][28][29] etc.
Due to its exceptional electrical, mechanical, and other properties, graphenea two-dimensional thick carbon atom organized in a honeycomb latticehas drawn a lot of interest for its possible uses in sensors, [30][31][32][33] catalysis, [34][35][36][37] energy storage devices, [38][39][40][41][42] and the environment.4][45][46] Multiple oxygen-containing functional groups are produced in GO and rGO by chemical oxidation modication techniques, offering a possible low-cost method for large-scale manufacturing of graphene-based products.][59][60] Aerogels based on graphene are among the world's lightest materials.6][67][68] However, the aerogel used to treat water environments is difficult to recover.3][74] Magnetic Fe 3 O 4 is combined with the initial materials to create magnetization for the material.5][76] The magnetic material is quickly recovered and reused aer removing toxic compounds from the water.
In this work, co-precipitation and partial reduction approaches were utilized to fabricate magnetic Fe 3 O 4 /graphene aerogel composite.Nanocomposites have been used to remove pesticide 2,4-dichlorophenoxyacetic acid from aqueous solution.

Preparation of graphene oxide
Using the enhanced Hummer process, graphite was converted into graphene oxide (GO) as follows: Aer adding 3 g of graphite powder to 42 mL of concentrated H 2 SO 4 , cool the mixture to 0 and 5 °C and stir consistently for 30 minutes.0.45 g of KMnO 4 was gradually added to the reaction mixture and agitated continuously for 15 minutes.Aer that, the mixture was agitated for 30 minutes while the temperature was kept at no higher than 35 °C and 9 g of KMnO 4 was added.The combination is gradually stirred, distilled water is added, and the system temperature is kept at or below 50 °C for an hour.10.5 mL of 30% H 2 O 2 were added following the reaction in order to eliminate any leover manganese dioxide and permanganate.Aer ltering, the substance was cleaned to a pH of 7 using distilled water and diluted HCl acid.The product was vacuum-dried at 60 °C.Using an agate mortar, the dried substance was ground ne and put in a sealed jar for storage. 77

Characterization
Applying an X'Pert Pro X-ray diffractometer (XRD) with a CuKa anode and a 2q range of 10°to 70°, the phase of the material was closely analyzed at a rate of 5°min −1 .The morphology of Fe 3 O 4 / GA was investigated employing scanning electron microscopy (SEM).Fe 3 O 4 /GA was calculated using energy-dispersive X-ray spectroscopy (EDX).The porosity of the materials was comprehensively examined at 77 K using the nitrogen gas adsorption isotherm (BET).FTIR infrared spectroscopy was employed to investigate the material's bonds and functional groups in the 400-4000 cm −1 wavenumber region.Magnetic force on a VSM device with a range of −10 000 to 10 000 Oe was used to assess magnetic saturation.

Adsorption of 2,4-dichlorophenoxyacetic acid
A constant amount of Fe 3 O 4 /GA (0.02 g) was added to 20.0 mL of an aqueous solution containing a predened amount of 2,4-D in order to conduct batch adsorption testing.All of the test parameters, adsorbent concentration, nanocomposite dosage, contact time, and pH, were varied while the experiments were carried out at room temperature (27 °C).The adsorption isotherm and kinetics were measured at 160 rpm and 30 C at predetermined intervals while the asks were being agitated in a circular shaker.Aer the adsorption procedure is complete, lter the mixture and use high-performance liquid chromatography on the Agilent Technologies of America 6340 Triple Quad LC/MS to measure the 2,4-D content.Sample measurement conditions were established: Zorbax Eclipse Plus C18 column 2.1 × 50 mm; V sample : 1.8 mm; mobile phase solvent A: 0.05% acetic acid, B: acetonitrile; ow rate: 0.26 mL min −1 .
The following formulas were used to determine the material's 2,4-D adsorption capacity and removal efficiency: where: V is the volume of 2,4-D solution (L); C 0 , C t are the concentrations of before and aer treatment 2,4-D solutions, respectively (mg L −1 ); m is the mass of Fe 3 O 4 /GA (g).Effect of time.Adsorb 20 mL of 2,4-D solution at a concentration of 100 mg L −1 with 0.02 g of Fe 3 O 4 /GA material shaking at 30, 60, 120, 150, 180, 210, 240, and 300 minute intervals.
Effect of pH.In general, a solution's pH has a signicant impact on the physicochemical processes occurring at the water-solid interface.The effect of pH on 2,4-D adsorption capacity was examined using 0.02 g of Fe 3 O 4 /GA material adsorbing 20 mL of 50 mg L −1 2,4-D solution at various pH values (3, 4, 5, 6, 7, 8, 10).The pH was adjusted using solutions of 1 M HCl and 1 M NaOH.
Effect of initial concentration.The adsorption effectiveness of the adsorbents is inuenced by the initial concentration of the simulated dye solution.20 mL of 2,4-D solution and 0.02 g of Fe 3 O 4 /GA were mixed at starting concentrations of 10, 30, 50, 70, 90, and 110 mg L −1 in order to study adsorption.

Results and discussion
The ndings of the X-ray diffraction analysis of the generated materials are displayed in Fig. 1a.The sharp diffraction peak (002) at 2q = 11.16°islinked to GO. 78,79 The XRD patterns of GO and  80,81 It's crucial to recollect that the (002) peak of GO did not show up during chemical reduction with ascorbic acid, but a broad peak about 24.6°b etween 20°and 30°did show up, suggesting that the Fe 3 O 4 nanoparticles were successfully coated on the graphene surface.The lack of a distinct peak at about 10°could potentially be attributed to the exfoliation of graphite oxide's layered structure.The partial rearranging of the delaminated graphene layers to produce a crystalline structure could be the reason for the large peak at around 24.6°. 78,82sing a superconducting quantum inference device, the magnetic characteristics of Fe 3 O 4 /GA were determined at ambient temperature with an applied magnetic eld ranging from −10 000 to 10 000 Oe.The magnetization curve versus applied magnetic eld in Fig. 1b illustrates the ferromagnetic characteristic of Fe 3 O 4 /GA.The hysteresis curve of the material is shown in Fig. 1b.Magnetic iron oxide nanoparticles have a saturation magnetism of M s = 20.66 emu g −1 .It proves that the magnetism of the Fe 3 O 4 /GA material is relatively high and can quickly separate from the sample solution through an external magnet, which can be used to recover materials in the eld of environmental treatment promptly.
The SEM images shown in Fig. 2 indicates that the GO material (Fig. 2a) has a moderately porous base without many voids.The morphology changed aer incorporating magnetic iron oxide into graphene and changing the state from hydrogel to aerogel.The Fe 3 O 4 /GA material has many pores and magnetic iron oxide particles with particles size of less than 100 nm dispersed on the surface of transparent graphene layers.A thin-lm interconnected porous 3D graphene is depicted in the presented SEM images of GO (Fig. 2a) and Fe 3 O 4 /GA (Fig. 2b), and Fe 3 O 4 nanoparticles are uniformly distributed on the surface of the ultra-thin graphene layer.
The samples characterized by FTIR infrared spectra shown in Fig. 3a are the corresponding GO, Fe 3 O 4 /GA spectra.In the region, 3500 cm −1 can be said to be the O-H group. 82The adsorption peaks at 1720 cm −1 correspond to the carbonyl functional groups at GO sheets' edges (COOH and C]O). 79The C]C vibrations of the graphene framework are thought to be responsible for the peak at 1620 cm −1 .At 1410 cm −1 and 1060 cm −1 , C-O vibrations in hydroxyl or epoxy groups were detected.One thing to notice is that, in comparison to the intensities in GO, all of the peak intensities at the vibrations of the oxygen functional groups in Fe 3 O 4 /GA are lower.This suggests that while the decrease of GO with ascorbic acid was Table 1 The isotherm models' linear equations

Isotherm model Equation
Langmuir Dubinin-Radushkevich ln q e = ln q m − b3 2        The results show that the material has good adsorption capacity in acidic environments around pH 3-5 and almost does not adsorb 2,4-D in alkaline environments around pH $ 8.The reason is when pH > pH pzc (7.6) Fig. 5b, the alkaline environment on the material's surface will have a negative charge, adversely affecting the 2,4-D adsorption of Fe 3 O 4 /GA.In the pH range < pH pzc , the material surface will be positively charged.2,4-D exists mainly in ionic form, so it is benecial for adsorption.In addition, in an alkaline environment, there is competition for the adsorption of OH − , so the adsorption capacity of the material was reduced.
The ndings demonstrated that at 240 minutes, the Fe 3 O 4 / GA's adsorption load peaked at 75.46 mg g −1 (Fig. 6a).Fig. 6b shows the effect of the 2,4-D concentrations on the adsorption efficiency.It can be seen that the adsorption efficiency is higher than 50% with 2,4-D concentrations of from 10 to 50 mg L −1 .The adsorption efficiency decreased signicantly when the, 4-D concentrations increases from 50 to 110 mg L −1 .At rst, the surface and capillaries are empty, and not much surface space is occupied.Furthermore, 2,4-D molecules are easily able to bind and penetrate.When the surface area occupied is large enough, the adsorption process takes place slowly and tends to increase slowly.Thus, the 2,4-D concentrations in range of 10 to 50 mg L −1 are considered appropriate concentration for the removal of 2,4-D by the Fe 3 O 4 /GA nanocomposite.
From the Langmuir, Freundlich, Tempkin, Dubinin-Radushkevich isothermal adsorption linear models of the material described in Fig. 7.We have calculated the parameters of the 2,4-D adsorption models of material Fe 3 O 4 /GA as shown in Table 2.According to the Freundlich model, K F coefficient = Table 3 Some features of putative firstand second-order kinetic models that differ from the 2,4-D experiment C 0 (mg L −1 ) q e,exp (mg L −1 ) Pseudo-rst-order Pseudo-second-order Based on rstand second-order models, the kinetics of 2,4-D adsorption on Fe 3 O 4 /GA were constructed at 2,4-D concentrations of 7, 14, 30, 48, and 67 mg L −1 , as illustrated in Fig. 8. Table 2 summarizes the adsorption rate constant and adsorption capacity determined using the rstand second-order apparent adsorption kinetic linear equations.The ndings demonstrate that the second-order apparent kinetic model is consistent with the 2,4-D adsorption process on Fe 3 O 4 /GA.
The following formula represents the substance's adsorption rate in the pseudo-second-order kinetic model: v = k 2 q e 2 (mg g −1 min −1 ) is shown in Table 3.When the adsorbent concentration is high, the amount of contact between the toxicant and the adsorbent surface increases.Therefore, the efficiency and adsorption rate increase.
According to the Langmuir model, the maximum adsorption capacity of 2.4-D by Fe 3 O 4 /GA was calculated to be 45.872 mg g −1 , which is comparable to other absorbents that have been previously reported (Table 4).Fe 3 O 4 /GA, on the other hand, has a more affordable production method and is readily scalable.According to this study, Fe 3 O 4 /GA may be applied more frequently in real-world scenarios than other adsorbents.

Conclusions
In summary, the hydrothermal approach has been effectively applied in the synthesis of Fe 3 O 4 /GA.The magnetic iron oxide nanoparticles are uniformly shaped, spherical, and have an average particle size of between 50 and 100 nm.They are uniformly distributed on the GO aerogel basis.The material's magnetism reaches 20.66 emu g −1 .Fe 3 O 4 /GA has an excellent adsorption capacity and efficiency, according to research examining factors impacting its ability to remove 2,4-D from water.The ideal pH range for the 2,4-D additive is 5-6, and it takes 240 minutes to equilibrate.It climbs quickly at concentration ranges #50 mg L −1 and progressively falls when 2,4-D concentration >50 mg L −1 .The acquired experimental values are in agreement with the 42.918 mg g −1 maximum adsorption capacity of the Langmuir, Freundlich, and Temkin isothermal adsorption theoretical model.A multimodal adsorption method, involving physical and chemical electrostatic interaction forces resulting from p-p interactions and hydrogen bonds, is employed to remove 2,4-dichlorophenoxyacetic acid from the material.The pseudo-second-order kinetic model describes the process kinetics.Using a magnet to extract the Fe 3 O 4 /GA from the solution is a practical and convenient way to utilize its strong magnetism.
Fe 3 O 4 /graphene aerogel (Fe 3 O 4 /GA) Disperse 0.8 g GO into 95 mL of a solution mixture containing 1 mM FeCl 3 $6H 2 O and FeCl 2 $4H 2 O. Stir the mixture in the induction cooker while slowly adding 2 M NaOH solution until pH ∼ 10.Ultrasound the system for 30 minutes and heat to about 80 °C.Add 0.8 g C 6 H 8 O 6 , and continue sonication for 20 minutes.To create Fe 3 O 4 /graphene hydrogel (Fe 3 O 4 /GH), transfer the mixture to a thermos and heat it at 90 °C for six hours.To obtain Fe 3 O 4 /GA material, sample Fe 3 O 4 /GH was refrigerated at −30 °C for 6 hours and then freeze-dried at −50 °C for 48 hours.

Fig. 1 Fig. 2
Fig. 1 The XRD diffraction pattern (a) and hysteresis curve (b) of GO and Fe 3 O 4 /GA materials.
successful, it was not total.In addition, because Fe 3 O 4 covers the GO surface, the reduction process still needs to be completed.The Fe-O vibration of Fe 3 O 4 has a distinctive vibration of approximately 590 cm −1 , especially in the FTIR spectrum.The intensity of oxygen-containing groups decreased for Fe 3 O 4 /GA, suggesting partial reduction of GO to create rGO.At 590 cm −1 , the Fe-O group appeared, signifying the bonding of ferromagnetic nanoparticles to the GA structure.The EDX spectra of Fe 3 O 4 /GA are shown in Fig. 3b.The presence of C, O, and Fe components in the mixture was conrmed by EDX spectra used to determine the elemental composition of Fe 3 O 4 / GA.Their existence suggests that the resultant goods are highly pure or do not have any signicant sources of contaminants (<0.5% -EDX technical limit of measurement).The EDX mapping analysis was employed to conrm the distribution of the Fe 3 O 4 nanoparticles on the surface of GA.The results is shown in Fig.4.It can be obvious that the Fe

Table 2 2
,4-D adsorption models' parameters of Fe 3 O 4 /GA Isotherm models Parameters Langmuir K L (L mg −1 ) 0.1284 q max (mg g −1 ) 42.918 R 2 0.9954 Freundlich K F (mg g −1 ) (L mg −1 ) Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 22304-22311 | 22307 Paper RSC Advances element are evenly distributed on the surface of the material.EDX spectrum analysis of Fe 3 O 4 /GA material in which the composition of elements C, O, and Fe are 65.16%, 21.37% and 13.47%, respectively.Fe 3 O 4 nanoparticle distribution on the carbon aerogel's surface is demonstrated by the Fe 3 O 4 /GA's EDX mapping spectrum.The reduction process also took place as a result of the material's surface having a lower fraction of O atoms.

Table 4
The 2,4-D adsorption capability of different adsorbents